Fusible alloy for pressure relief devices

ABSTRACT

The present invention relates to a fusible alloy for pressure relief devices (PRDs), and the present invention provides a fusible alloy for PRD, the alloy comprising 29.0 to 33.0% by weight of Bi, 14.0 to 21.0% by weight of Sn, 2.0 to 5.0% by weight of In, and substantially Pb for the balance. 
     The fusible alloy for PRD of the invention has a melting point appropriate for use according to the 110° C.-grade standards, and has excellent wettability upon melting.

TECHNICAL FIELD

The present invention relates to a fusible alloy for pressure reliefdevices (PRDs), and more particularly, to a fusible alloy for PRD, whichis appropriate for use according to the 110° C.-grade standards and hasexcellent wettability upon melting.

BACKGROUND ART

Pressure relief devices are safety devices attached to gas cylindervalves in automobiles and the like, which devices play the role ofpreventing gas explosion by rapidly discharging gases inside thecylinder to the outside when the temperature of the ambient environmentrises abnormally to above a specific temperature, such as at the time offire.

Accordingly, a conceptually ideal PRD is expected to completely cut offthe outflow of gases up to a specific temperature, and when the specifictemperature is reached, to discharge the entire amount of the gaseswithin a short time. Thus, only those verified products which haveundergone rigorous tests are put into use.

In this light, for example, the European standards stipulate a use ofalloys having a melting point acceptable for the 110° C.-grade (110±10°C.), to provide against safety accidents involving gas explosions.

DISCLOSURE

As such fusible alloys for PRD, ternary alloys such as Bi—Pb—Sn alloys,Bi—Cd—Sn alloys and the like have been traditionally used. However, thewettability upon melting of these alloys is not sufficient, and thealloys have had limitations in rapidly discharging gases inside thecylinder to the outside.

Thus, it is an object of the present invention to solve such problems ofthe prior art, and to provide a new fusible alloy for PRD, comprising aquaternary alloy which ensures excellent wettability upon melting at aspecific temperature, and at the same time, meets the requirementsstipulated by the European standards as the 110° C.-grade.

DESCRIPTION OF DRAWINGS

FIG. 1 shows photographs showing the wetting angles of the fusible alloyfor PRD according to the present invention and of a conventional fusiblealloy for PRD, as measured in a wettability test.

FIG. 2 shows scanning electron microscopic (SEM) photographs ofexemplary reinforcement materials used in the preparation of the fusiblealloy according to the present invention.

BEST MODE

In order to achieve the above-described object, according to the presentinvention, there is provided a quaternary fusible alloy for PRD,comprising 29.0 to 33.0% by weight of bismuth (Bi), 14.0 to 21.0% byweight of tin (Sn), 2.0 to 5.0% by weight of indium (In), andsubstantially lead (Pb) for the balance.

The present invention chooses to use a quaternary alloy because thelow-melting point alloying elements, namely, Pb, Bi, Sn and In,themselves have low melting points, and also because when these elementsform a polynary alloy, the alloy may have a eutectic point, thus itbeing possible to render the melting point of the alloy even lower. Forexample, in the case of a Pb—Sn alloy, which is a representative soldermaterial, while the melting point of Pb is 327° C., and the meltingpoint of Sn is 232° C., an alloy of Pb with 61.9% of Sn has a meltingpoint of 183° C. at the eutectic point. Based on this tendency, themelting point of an alloy can be further lowered by increasing thenumber of alloying elements to systems of ternary, quaternary and thelike.

Meanwhile, according to the present invention, the results ofthermodynamic calculations were used as the basis of the primary alloydesigns for appropriate composition ratios of the alloying elements. Inother words, the melting points of ternary, quaternary and quinaryalloys of various compositions were predicted based on the calculationsusing thermodynamic data. Then, a quaternary system of Bi—Sn—In—Pb thatwas considered to be an optimum was selected to constitute the alloy ofthe present invention, and the composition was determined to fall withina specific range through actual experimentation.

Thus, in the present invention, the composition ratios of thealloy-constituting elements were determined to be 29.0 to 33.0% byweight for Bi, 14.0 to 21.0% by weight for Sn, 2.0 to 5.0% by weight ofIn, and substantially Pb for the balance. If the respective componentelements are contained in the alloy of the invention in amounts beyondthe above-described composition ranges, the melting point (inparticular, the melting start point) may be lowered to below 100° C.,thus causing the alloy to melt at a temperature below the criticaltemperature; or the melting point may be raised to above 120° C., thusinhibiting efficient discharge of gases at an appropriate temperatureand subsequent prevention of safety accident such as gas explosion.Then, such alloys are deprived of the function as the 110° C.-gradefusible alloys for PRD, and are undesirable.

Hereinafter, the present invention will be described in more detail.

Evaluation of Main Properties of Fusible Alloy for PRD

The properties that are necessary for an alloy to be used as a fusiblealloy for PRD, include melting point, wettability, PRD durability, andthe like.

Evaluation of Melting Point In order to prepare alloys of desiredcompositions, the composition ratios by weight were calculated, and theelements were respectively weighed. Then, the elements were mixed andmelted together to form fusible alloys, and then, mother alloy ingots ofthe respective compositions, each weighing about 1 kg, were prepared.Subsequently, each fusible alloy was placed on a substrate made of brassand maintained at a desired temperature for 10 minutes using glycerinsolution as a heating medium, and then, the presence or absence ofmelting of the fusible alloy was verified. The compositions of thealloys were analyzed by energy dispersive spectroscopy (EDS) (an averageof three points), and the evaluation results are presented in Table asthe melting point according to the composition of an alloy.

TABLE 1 Melting Composition (ratio start Melting by weight, wt %) pointend point Pb Bi Sn In (° C.) (° C.) Example 1 48.85 29.38 18.85 2.92113.0 118.0 2 48.36 29.25 19.46 2.94 109.0 117.0 3 46.81 30.96 18.283.95 106.0 116.0 4 45.77 30.87 19.87 3.50 112.0 120.0 Comp. Example 142.15 28.41 25.97 3.47 98.4 117.0 2 17.31 71.08 9.90 1.70 98.0 100.0 322.62 60.71 13.08 3.59 99.2 107.0 4 30.43 47.19 16.03 6.36 99.4 116.0

As can be seen from Table 1, the alloys having the compositionsaccording to the present invention (Examples 1 to 4) were all found tohave melting points that exceed 100° C., thus satisfying the Europeanstandards relating to the 110° C.-grade fusible alloys for PRD. On theother hand, the alloys of Comparative Examples all had melting startpoints of 99.4° C. or lower, and were found to be undesirable sincemelting of the alloys would start at a temperature lower than theminimum required temperature (in the case of the 110° C.-grade, theallowed temperature range is 110±10° C., and thus, 100° C. is theminimum required temperature).

Evaluation of Wettability

In order for a fusible alloy to form strong bonding to the internalwalls of a PRD and the interior reinforcement materials, the fusiblealloy should have excellent wettability. A wettability test is performedby melting alloys of the same weight and measuring the area occupied bythe alloy, or by inspecting the cross-section of a specimen that hasbeen melted and quenched, and measuring the wetting angle. A smallerwetting angle means better wettability. FIG. 1( a) in the upperphotograph shows a conventional Bi—Cd—Sn ternary alloy, and FIG. 1( b)in the lower photograph shows a Pb—Bi—Sn—In quaternary alloy (Example 4)according to the present invention.

Referring to FIG. 1, the wetting angle of the Bi—Cd—Sn ternary alloy wasmeasured to be 52.78°, while the wetting angle of the Pb—Bi—Sn—Inquaternary alloy of the present invention was measured to 47.25°. Thus,the wetting angle of the alloy of the present invention was confirmed tobe smaller.

Evaluation of PRD Durability

To evaluate the durability of a PRD, a PRD needs to be actually producedfrom a fusible alloy. A PRD employing a composite material is formed byfilling the interior of the PRD body with a fusible alloy mixed with thereinforcement materials as shown in FIG. 2. The reinforcement materialsused in the present invention were stainless steel spheres having adiameter of about 600 mm (FIG. 2( a)) and stainless steel cylindershaving a length of about 900 mm (FIG. 2( b)), which were used in amixture. The experimental results indicate that when the reinforcementmaterials were mixed at a ratio of about 50:50, the durability was foundto be optimum.

As such, PRDs were produced using the quaternary fusible alloysaccording to the present invention, and a durability test was performed.According to the test conditions stipulated by the ISO InternationalStandards, the test temperature was 91° C., the test pressure was 325bars, and the test duration was 500 hours. After 500 hours of the test,if the fusible alloy inside the PRD did not flow out, the fusible alloywas considered to be acceptable. The test procedure is as follows.

(1) A sample PRD is placed in a test block, which is in turn placed inan oven.

(2) A pressure of 325 bars is applied onto the sample PRD by means ofair.

(3) The temperature in the oven is adjusted to 91° C., and then thesample PRD is maintained therein for 500 hours.

(4) No deformation or operation of the sample PRD is allowed for theduration of 500 hours.

When the test was performed in the above-described manner, those PRDsfilled with the fusible alloys of the present invention were all foundto be acceptable, and were confirmed to have durability that is at leastequivalent to that of existing alloys.

INDUSTRIAL APPLICABILITY

As discussed in the above, the quaternary fusible alloy for PRD of thepresent invention has a melting point that is appropriate for useaccording to the 110° C.-grade standards, and has excellent wettabilityas demonstrated by the small wetting angle upon melting. Thus, PRDsformed of the fusible alloy of the invention can be expected todischarge gases within a short time, and the fusible alloy of theinvention is a material which can satisfactorily replace conventionalternary alloys.

1. A fusible alloy for pressure relief devices, the alloy comprising29.0 to 33.0% by weight of bismuth (Bi), 14.0 to 21.0% by weight of tin(Sn), 2.0 to 5.0% by weight of indium (In), and substantially lead (Pb)for the balance.